Ceramic discharge vessels are generally used for high-intensity discharge (HID) lamps which include high-pressure sodium (HPS), high-pressure mercury, and metal halide lamp types. The ceramic vessel must be translucent and capable of withstanding the high-temperature and high-pressure conditions present in an operating HID lamp. The preferred ceramic for forming discharge vessels for HID lamp applications is polycrystalline alumina (PCA), although other ceramics such as sapphire, yttrium aluminum garnet, aluminum nitride and aluminum oxynitride may also be used.
In conventional ceramic discharge vessels, conductive metallic feedthroughs are used to bring electrical energy into the discharge space. However, making the hermetic seal between the ceramic vessel and the metallic feedthrough can be troublesome because of the different properties of the materials, particularly with regard to the thermal expansion coefficients. In the case of polycrystalline alumina, the seal typically is made between the PCA ceramic and a niobium feedthrough since the thermal expansion of these materials is very similar. The niobium feedthrough is joined with at least a tungsten electrode which is used to form the point of attachment for the arc because it has a significantly higher melting point compared to niobium.
Niobium however as a feedthrough material has two significant disadvantages. The first disadvantage is that niobium cannot be exposed to air during lamp operation since it will oxidize and cause lamp failure. This necessitates that the discharge vessel be operated in either a vacuum or inert gas environment, which increases cost and the overall size of the lamp. The second disadvantage is that niobium reacts with most of the chemical fills used in metal halide lamps. Although the results of this reactivity are varied, these reactions inevitably lead to reduced lamp performance or life.
This concern has lead to the development of more complex electrode assemblies for metal halide applications. For example, one prior art electrode assembly for a ceramic metal halide lamp is comprised of four sections welded together: a niobium feedthrough for sealing to the ceramic arc tube; a molybdenum rod; a Mo-alumina cermet, and a tungsten electrode. Another described in U.S. Pat. No. 6,774,547 uses a multi-wire feedthrough having a ceramic core with a plurality of grooves along its outside length with the wires inserted in the grooves. The wires, either tungsten or molybdenum, are twisted together at least at one end of the feedthrough. The twisted wire may be used as the electrode inside the lamp or a separate electrode tip may be attached to the twisted wire bundle.
U.S. Pat. No. 4,366,410 describes closure members made from Mo—Ti and Mo—V alloys in place of niobium. The Mo—Ti and Mo—V alloys can be formulated to have coefficients of thermal expansion to match PCA. In addition, U.S. Pat. No. 4,334,628 further teaches that up to 5 weight percent of a sintering aid (Ni, Co or Cu) may be added to a Mo—Ti alloy to facilitate fabrication of the closure member by sintering. Unfortunately, both of these molybdenum alloys also have disadvantages. In particular, the Mo—Ti alloys adversely react with the metal halide chemical fills and the Mo—V alloys are very brittle and difficult to manufacture.